In a pyro-sensor such as oxygen and NOx sensors in which a sensing portion is heated by the heater, the characteristics of the pyro-sensor will be changed depending on a heater's itself temperature variation or a temperature variation of the heater due to the temperature variation around the environment. For example, a platinum thin film heater changes its resistance in response to temperature as shown in T (temperature)-R (resistor) characteristics of FIG. 1.
As a conventional heater temperature control method, for example, JP-A-60-114758 official gazette has been proposed to maintain temperature of the pyro-sensor to high temperature value. This method measures an environmental temperature by disposing a temperature sensor such as thermocouples or a thermistor adjacent to the sensor, and a load voltage to the heater is controlled as the result to maintain a sensor temperature to a predetermined value. In this case, the temperature sensor that ambient air temperature is only measured should be necessary, and a sensor's structure becomes much more complex, and it results in an expensive cost.
FIGS. 2 and 3 show another heater for the pyrosensor and its pyro-control circuit disclosed in Japanese Patent Application No. 8-298267 that is filed by the inventor on Oct. 22, 1996. In FIG. 2, a main heater 30 of platinum thin film is mounted on an insulator substrate 2 in alignment with a detective or sensing area, and an auxiliary heater 32 of platinum thin film is also arranged inside the main heater 30.
In FIG. 3, a collector of a transistor 20 is connected to the positive line voltage +Vc line, and its emitter is grounded through a bridge circuit 18 and the main heater 30. A first resistor 12 of the bridge circuit 18 is serially connected to the auxiliary heater 32 at a node 13, and its second resistor 14 is serially connected to its third resistor 16 and a variable resistor 34 through a node 17.
These nodes 13 and 17 are respectively connected to inputs of an operational amplifier 24. An output of the amplifier 24 is connected to the base of the transistor 20 through a protection resistor 22. The resistor 36 for supplying an early voltage to the bridge circuit 18 upon triggering is connected between the collector and emitter of the transistor 20.
In FIG. 3, the transistor 20 is so controlled by the amplifier 24 that the electric potential e1 of the node 13 is identical to that e2 of node 17 to maintain the temperature of the auxiliary heater 32 to a fixed or predetermined value.
For example, when the temperature of the auxiliary heater 32 is lower than the predetermined temperature value or e1&lt;e2, the output voltage of the amplifier 24 is raised, and the main heater 30 and the auxiliary heater 32 are further heated to maintain the predetermined temperature condition.
In FIG. 2, the leakage current may flow to the auxiliary heater by the potential difference induced between the main and auxiliary heaters when the insulation of the insulator substrate arranged on the main and auxiliary heaters is deteriorated by, for example, aging. Therefore, though a configuration of the conventional pyro-control circuit is simple, the electric potential e1 will be increased by the leakage current, and the temperature of the auxiliary heater 32 has a problem to decrease less than the predetermined value.
FIG. 4 shows a block diagram of another conventional pyro-sensor. In FIG. 4, a common power supply 46 is provided to heat a heater 42 in a sensor 40 through first power supply 43, and to supply an applied voltage to a cell 44 in the sensor 40 through second power supply 45.
FIG. 5 is a cross-sectional top view to show an example of the sensor 40. In this drawing, a platinum thin film heater 42 is mounted or affixed on, for example, one surface of an insulator substrate 50, and its entire surface is covered with an insulating material 51 having the heat proof and corrosion tolerant characteristics. A cell substrate 55 for supporting the cell 44 is mounted on other side of the insulator substrate 50 through an insulation spacer 53.
When the above common power supply 46 drives the sensor 40 having, for example, a structure of FIG. 5, the deterioration or contact of the insulation characteristics between the heater 42 and the cell 44 may be generated by aging, and it is afraid that an unusual leakage current flows therebetween. This leakage current degrades the characteristics of the fresh sensor, and it is one of the causes which make reliability decline.
In other words, in the limit current type oxygen sensor having the structure like FIG. 5, the insulation characteristics between the heater 42 and the cell 44 is supposed to be degraded, because first power supply 43 and second power supply 45 are directly connected to the common power supply 46, a leakage current is generated regardless of more or less therebetween, one current loop (see FIG. 6) is generated through common ground both of power supplies. Then, the output of the sensor go wrong or the deterioration and reliability of the sensor is degraded.
Accordingly, it is an object of the invention to provide a high-reliable heater for the pyro-sensor and its pyro control circuit in which any leakage current doesn't occur even if the electric insulation of the insulator substrate deteriorates by aging, and then the main heater can maintain a predetermined temperature value as well as the auxiliary heater that serves as a temperature sensor.
It is another object of the invention to provide a method for driving a high-reliable sensor in which any leakage current from the heater is not generated, even if insulation characteristics between the heater and cell is degraded.
It is still another object of the invention to provide a driving method of the high reliable pyro-sensor that a current loop doesn't appear between the heater and the cell.
A main heater 30 consisting of first fever area 30a and second fever area 30b, and an auxiliary heater 32 are arranged on an insulator substrate to maintain a sensing portion, for example, 400.degree. C. A grounded first fever area 30a is contiguously arranged between second fever area 30b and grounded auxiliary heater 32. An amplifier 24 controls a load voltage to a bridge circuit 18 containing the auxiliary heater 32, first, second and third resistors 12, 14, 16 and the main heater 30 based on the output of the bridge circuit 18. An inverting input of an amplifier 23 is connected to a node 13, and a non-inverting input thereof is connected to a node 17 of second and third resistors 14 and 16, and an output terminal is connected to the base or gate of the transistor 20 supplying the load voltage to the main heater 30. The resistance ratio of first fever area 30a and second fever area 30b is determined to be identical to that of third resistor 16 and second resistor 14. Pyro-sensor or limit current type oxygen sensor comprises a cell 44 capable of maintaining a high temperature, for example, 400.degree. C. and a heater 42 for heating the cell 44 to the predetermined temperature. It firther comprises a DC-DC converter 48 isolating a first power supply 43 supplying electric power to the heater 42 from second power supply 45 applied a voltage to the cell 44.
A leakage current by deterioration of the insulator substrate is not generated, and temperature of a sensing portion is constantly maintained by the main heater to a high temperature. The sensor cell is protected from a leakage current from the heater from occurring.
A heater for the pyro-sensor according to the present invention is characterized in that a main heater consisting of first and second fever areas, and an auxiliary heater are arranged respectively on an insulator substrate; and first fever area is contiguously arranged between said second fever area and said auxiliary heater.
Therefore, for example, a common ground terminal connected to first fever area and the auxiliary heater is provided on one side or adjacent to the corner of a rectangular insulator substrate, and a power supply voltage is applied to a load terminal connected to second fever area so that the electric potential distribution or field between these heaters may be identical thereto.
According to the invention, there is provided a pyro-sensor comprising a cell and a heater for heating said cell in predetermined temperature, and characterized in that a first power supply supplying electric power to said heater is isolated from second power supply applying a voltage in said cell.
Said pyro-sensor is a limit current type oxygen sensor, and said first power supply is characterized by isolating from said second power supply through an insulation type DC-DC converter. Especially, the insulation type DC-DC converter is provided as first power supply to isolate first power supply from second power supply. The circuits relating to the heater and the cell are performed independent systems, respectively.
Because the current loop of second power supply 45 is insulated from that of first power supply 43 by the DC DC converter 48, specially, a transformer, any current never flows from the heater's loop to the cell's loop or vise versa, any leakage current doesn't appear and reliability of the sensor is improved even if insulation characteristics between the cell and the heater is degraded.